The invention relates to a method and a gas sensor for selective gas detection, as well as for measuring the corresponding gas concentration through the utilization of light beams in the near infrared region.
There is a great need for cost-effective and reliable gas sensors for a number of tasks in the area of safety, comfort and environmental protection. In particular, the air has to be monitored for gas concentrations which are explosive, toxic or uncomfortable to humans. The improved characteristics of the DFB laser diodes (distributed feed back) available today can be advantageously used in optical detection in the near infrared region. The material system InGaAsP (Indium Gallium Arsenid Phosphid) enables the manufacture of laser diodes in the wavelength region between 1.1 and 2.0 xcexcm. The spectral single-mode DFB laser diodes, which can also meanwhile be manufactured for operating temperatures up to 100xc2x0 C., are particularly suitable for gas detection. As for the gasses, there exist molecules which comprise absorption bands in the near infrared region. There are, for example, H2O, CO, CO2, NH3, HF, CH4, HCL, NO2, O2 which should be mentioned here.
Specific hydrogen-containing molecules such as methane (CH4) show a relatively strong absorption in this wavelength region, which simplifies the technical feasibility of the detection and thus favors the application of laser diodes in the near infrared region for methane detection.
For safety in contact with furnaces driven by natural gas and with stoves in the domestic area, as well as in underground working and the development and supplying of natural gas, there is a need for methane sensors which are in the position to detect with certainty methane concentrations which are far below the ignition threshold (5 Vol %). For this, a detection threshold between 3 and 20% (corresponding to 0.15-1 Vol %) of the lower ignition limit is required. The following norm applies thereto: xe2x80x9cElectric apparatus for the detection of combustible gases in domestic premisesxe2x80x9d, Europxc3xa4ische Norm, Final Draft prEN 50194, May 1995. Just as important as the certain detection of the methane is the exclusion of false alarms due to interfering gasses or due to altering events of the sensor. In addition, the freedom from maintenance and long-term stability of the calibration are indispensable for utilization in the private household.
Methane detection is currently carried out with solid-state sensors. Currently available sensors are inadequate for the following reasons. Pellistor gas detectors, which detect on the basis of a catalytic combustion of the methane, are susceptible to certain interfering gasses such as silicon-containing gasses, for example. The interfering gasses effect a sensitivity loss and thus call into question the certainty of detection.
Metal oxide detectors are also utilized for methane detection. The most frequently used material for this is tin oxide. These sensors are generally cross-sensitive to other reducing gasses and also to air moisture, which can lead to false alarms. Reducing gasses are, for example, alcohol, propane/butane as fuel gas for gas cylinders, or volatile organic gasses. However, false alarms result in gas warnings no longer being taken seriously. The combination of a gas detection with an automatic shutoff of the gas supply in the case of an alarm leads to uncertainty and irritation of the user if the gas detection is unreliable and false alarms occur.
The maintenance-free lifetime of the known solid-state sensors lies between one and five years. The lifetime for a methane detector desired by the marketplace, however, is over ten years.
Besides the solid-state sensors, infrared absorption in the hydrocarbon band at 3 xcexcm wavelengths is also used. The wavelength selection ensues therein with an interference filter. Due to the opposing superposition of the absorption bands of the different hydrocarbons, this method does not permit a selective detection of methane, for example. In the absorption measurement in the near infrared region (NIR) with laser diodes, specific emphases are set. As a rule, the detection method concerns the smallest concentrations, this being connected with costly measurement arrangements. The following literature is cited in this regard: Y.Shimose, T. Okamoto, A. Maruyama, H. Nagai, xe2x80x9cRemote sensing of Methane Gas by Differential Absorption Measurement Using a Wavelength Tunable DFB LD, IEEE Photonics Technology Lettersxe2x80x9d, Vol 3, No.1, January 1991, 86-87; Kiyoji Uehara, Hideao Tai, xe2x80x9cRemote detection of methane with a 1.66 xcexcm diode laserxe2x80x9d, Applied Optics, Feb. 20, 1992, Vol. 31, No. 6, 809-814; and R. U. Martinelli, R. J. Menna, D. E. Cooper, C. B. Carlisle, H. Riris, xe2x80x9cNear-infrared InGaAs/lnP Distributed-Feedback Lasers for Spectroscopic Applications, Proc. Spie-In. Soc.Opt.Eng. (USA)xe2x80x9d, Laser Diode Technology and Applications VI, Vol 2148, 292-307, 1994.
A construction for detecting methane with laser diodes in the second harmonic band of methane at 1.325 xcexcm was proposed by M. T. Pichery, xe2x80x9cLes detecteurs de gaz domestiques par methode optique, Gaz d""aujourd""huixe2x80x9d, No. 6,1996, 271-273. Due to the low absorption strength in the second harmonic band, a multipath cell with a large absorption distance (larger than 1 m) is therein required. In this construction here described, a cost-effective construction of a sensor of this type remains at the forefront.
The concentration determination of gasses with infrared technology is based on the absorption measurement in the vibration-rotation bands of the gasses. For the optical absorption, the well-known Lambert-Beersche absorption law applies:
I("ugr")=I0xc2x7exe2x88x92xcex1("ugr")cxc2x7l
Therein, I0 is the irradiated intensity, c is the gas concentration, and xcex1 (xcexc) is the wavelength-dependent absorption coefficient. The absorption takes place inside the optical wavelength l. xcex1 (xcexc) is composed of the line width and absorption profile (Lorenz profile for normal pressure).
It is the object of the invention to make available a method and a gas sensor for the selective detection of given gasses, wherein the requirements with reference to the detection limits and the reliability are increased, and the sensor construction is cost-effective.
The invention is based on the principle that the narrow-band measurement of the absorption by means of laser light sources enables the spectrally resolved measurement of individual absorption lines and thus the selection of an individual gas component. The harmonics of the molecular vibrations lie in the near infrared region. With the material system InGaAsP, laser diodes which emit light in this region can be manufactured. The region of the near infrared light extends approximately from 0.65 to 2 xcexcm wavelengths. The spectral line width of the laser is less than 10 MHz and thus lies at 0.01 times the half-width value of a typical gas absorption line. The dependency of the emission wavelength of the cited laser diodes on the temperature (typically 0.1 nm/K), or respectively, on the laser stream or flux is used to pick up the spectrum of at least one absorption line. If two spectral lines lie relatively close together, then it can make sense to pick up more than one line. The spectrum can be compared to a theoretical calculation of the absorption line, wherein, as significant parameter, the gas concentration is obtained from the comparison. This method comprises two significant advantages. For one, a calibration of the measurement construction with test gasses is omitted. Secondly, this method is to a great extent independent of the transmission of the absorption distance (measured length), so that contaminations of optical windows have no effect on the measurement result.
Accordingly, the present invention provides a method for selectively detecting and measuring the concentration of a gas through light absorption in the near infrared region. The method includes the steps of selecting at least one spectral line of the spectrum of the gas to be detected and selecting a distributed feed back laser diode with an emission wavelength in the near infrared region that is dependent upon an operating temperature of the laser diode and that corresponds to the selected spectral line. The method further includes the steps of maintaining the laser diode at a temperature that is greater than or equal to 40xc2x0 C. and emitting light from the laser diode having an emission wavelength in the near infrared region. The light is emitted across a measured distance to a photodetector. Thereafter, the gas is detected by varying the operating temperature of the laser diode so that the emission wavelength of the light emitted from the laser diode overlaps the spectral line of the gas to be detected. The concentration of the gas is measured by measuring an absorption difference at the photodetector in a wavelength region that corresponds to the selected spectral line at two different operating temperatures of the laser diode and thereafter calculating the concentration of the gas to be measured from the absorption difference.
In an embodiment, the operating temperature of the laser diode is varied by utilizing an electrical resistance heating element which heats the laser diode.
In an embodiment, the operating temperature of the laser diode is varied by emitting light from the laser diode which heats the laser diode.
In an embodiment, the method further includes the step of callibrating the photodetector by measuring an absorption of a known reference gas of a known concentration and composition.
In an embodiment, the reference gas is disposed in a hollow enclosure directly in front of the laser diode so that light emitted from the laser diode passes through the reference gas before it reaches the photodetector.
In an embodiment, the reference gas comprises a known concentration of the gas to be measured and detected.
In an embodiment, the present invention provides a gas sensor which comprises a housing having two opposing ends with a laser diode and photodetector disposed at one opposing end and a hollow mirror disposed at an opposing end. Light emitted from the laser diode is reflected off of the hollow mirror and back to the photodetector. The middle portion of the housing has a defined distance and is filled with a gas to be measured.
In an embodiment, the housing further comprises a hollow enclosure filled with a reference gas. The hollow enclosure is disposed in front of the laser diode so that light emitted from the laser diode passes through the reference gas before it reaches the hollow mirror.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.